1. Technical Field
The present invention relates to electronic circuits, electro-optical devices, and electronic apparatuses including the same. More particularly, the invention relates to a circuit formed on a substrate using a thin-film polysilicon deposition technique, a liquid crystal display device including the circuit, and an electronic apparatus including the liquid crystal display device.
2. Related Art
In recent years, a technology in which various thin-film transistor circuits are formed on glass substrates using a low-temperature polysilicon deposition technique, i.e., a System On Glass (SOG) technology, has been actively developed. Examples include a monolithic driver in which a driver circuit is integrated onto a glass substrate of a liquid crystal display.
As one application of the SOG technology, it is conceivable to form a sensor and its sensing circuit on a glass substrate. For example, by using a structure in which an optical sensor and a sensing circuit are integrated onto a transparent substrate constituting a liquid crystal display, and in which the state of external light is detected, and the illuminance of a backlight is controlled on the basis of the detected results, it is possible to maintain the optimal visibility of the liquid crystal display regardless of the environment. There are many other sensors, such as temperature sensors, gyro sensors, and inclination sensors, that benefit from being disposed on glass substrates. In general, outputs of these sensors are analog signals. In order to process the analog signals by a digital logic circuit disposed on a glass substrate so as to be used for proper control, it is necessary to convert the signals to digital form with an A/D conversion circuit.
FIG. 12 is a circuit diagram showing an example of an A/D conversion circuit of a known current device. In the current device (sensor 1), one end is connected to a power supply voltage VD, and the other end is connected via a Node-A, the voltage of which is VA, to a drain electrode and a gate electrode of a transistor 2. A source electrode of the transistor 2 is connected to a power supply voltage VS. The Node-A is connected to a comparator circuit 3.
In the current device, which is the sensor 1, a current Isense, which flows between the power supply voltage VD and the Node-A, changes according to the quantity of a physical stimulus to be sensed and a voltage (VD−VA) applied between the power supply voltage VD and the Node-A.
In the transistor 2, the voltage between the gate and the source is given by the equation Vgs=VA−VS and the voltage between the drain and the source is given by the equation Vds=VA−VS. Hence, Vds=Vgs. Therefore, if the threshold value Vth of the transistor 2 satisfies the relationship Vth>0, the relationship Vds>Vgs−Vth is satisfied. When Vth<VA−VS, the transistor 2 always operates in a saturation region. The characteristics of a general MOS transistor in the saturation region are expressed by equation (1).
                    Ids        =                              W                          2              ⁢              L                                ×          μ          ×          C          ⁢                                          ⁢          0          ×                                    (                              Vgs                -                Vth                            )                        2                                              (        1        )            Here, W is the channel width of the transistor, L is the channel length, μ is the mobility, and C0 is the gate capacitance.
From Kirchhoff's law, the relationship Isense=Ids is obvious. Hence, equation (2) is satisfied.
                    Isense        =                              W                          2              ⁢              L                                ×          μ          ×          C          ⁢                                          ⁢          0          ×                                    (                              Vgs                -                Vth                            )                        2                                              (        2        )            
Equation (2) can be transformed into equation (3).
                    Vgs        =                                                            Isense                ×                2                ⁢                L                                            W                ×                μ                ×                C                ⁢                                                                  ⁢                0                                              +          Vth                                    (        3        )            
Since VA=Vgs+VS, equation (4) is derived.
                    VA        =                                                            Isense                ×                2                ⁢                L                                            W                ×                μ                ×                C                ⁢                                                                  ⁢                0                                              +          Vth          +          VS                                    (        4        )            
Assuming that the current Isense does not depend on the voltage VA, it is possible to easily obtain the current Isense from the voltage VA.
One example of a sensor device that satisfies the assumption is an optical sensor device which uses a PN junction diode or a PIN junction diode. When a reverse bias is applied to such a device, the current Isense is a constant current source which generates a current proportional to the light illuminance in a certain range. Hence, equation (5) is valid.
                    VA        =                                                                              2                  ⁢                  L                  ×                  Isense                                                  W                  ×                  μ                  ×                  C                  ⁢                                                                          ⁢                  0                                                      ⁢            Vth                    +          VS                                    (        5        )            
That is, the current Isense is calculated from the voltage VA according to equation (6).
                    Isense        =                                            (                              VA                -                Vth                -                VS                            )                        2                    ×                                    W              ×              μ              ×              C              ⁢                                                          ⁢              0                                      2              ⁢              L                                                          (        6        )            
Here, the voltage VA of the Node-A is input to the comparator circuit 3. FIG. 13 is a circuit diagram showing a configuration of the comparator circuit 3. This circuit compares the input voltage Vin with the reference voltage Vref. If Vin(=VA)>Vref, the circuit outputs a High voltage (≈VD) to an output signal Out. If Vin(=VA)<Vref, the circuit outputs a Low voltage (≈VS) to the output signal Out. Therefore, when the reference voltage Vref is applied to the comparator circuit 3, by referring to the digital output result of the terminal of the output signal OUT, it is possible to find out whether the voltage VA is higher or lower than the reference voltage Vref. In such a case, when digital conversion is desired in 256 gradations, 255 comparator circuits 3 are arranged in parallel, and different reference voltages Vref are applied thereto. Alternatively, it is possible to perform conversion in the same manner by inputting a staircase waveform (255 STEP) to the reference voltage Vref, and by storing the output results. Which method is to be selected may be determined depending on the circuit area, power consumption, and sampling rate. Furthermore, it is of course possible to employ a configuration in which both methods are combined. Namely, it is possible to perform digital conversion in 256 gradations by inputting 16 staircase waveforms (15 STEP) having different levels into 16 corresponding comparators.
Furthermore, in the circuit shown in FIG. 13, preferably, transistors 3a, 3b, 3c, and 3d have the same size. For example, in each of the transistors 3a, 3b, 3c, and 3d, the channel width W is set at 10 μm, and the channel length is set at 6 μm.
Since the voltage VA is determined in a certain range as described above, by using equation (6), it is possible to determine the current Isense, which gives the physical quantity to be determined, for example, illuminance. That is, the output of the sensor can be A/D converted.
Furthermore, even in a sensor in which the current Isense of the sensor 1 is not constant and is a function of the voltage (VD−VA), A/D conversion can be performed using the same electronic circuit. For example, in a sensor device having a certain impedance Rsense that depends on a measured quantity, the current Isense is expressed by equation (7).
                    Isense        =                              (                          VD              -              VA                        )                    Rsense                                    (        7        )            
Examples of such a device include a temperature sensor including a resistor, and a gyro sensor including a variable resistor. A sensor is taken as an example, in which the impedance Rsense varies with temperature according to equation (8).
                    Rsense        =                  R          ⁢                                          ⁢          0          ×                      exp            ⁡                          (                              B                T                            )                                                          (        8        )            Here, T is the absolute temperature (K), and R0 and B are characteristic constants of the temperature sensor.
The following equation (9) is obtained from equations (4) and (7).
                    VA        =                                                            2                ⁢                L                ×                                  (                                      VD                    -                    VA                                    )                                                            Rsense                ×                W                ×                μ                ×                C                ⁢                                                                  ⁢                0                                              +          Vth          +          VS                                    (        9        )            The above equation is solved to give equation (10).
                    Rsense        =                              2            ⁢            L            ×                          (                              VD                -                VA                            )                                            W            ×            μ            ×            C            ⁢                                                  ⁢            0            ×                                          (                                  VA                  -                  Vth                  -                  VS                                )                            2                                                          (        10        )            Thus, the impedance Rsense is calculated from the voltage VA. By substituting the resulting value into equation (8), the temperature T is obtained.
However, in general, characteristics, in particular, saturation characteristics, of polysilicon thin-film transistors are inferior to those of MOS transistors which are formed on single-crystal silicon substrates. Therefore, polysilicon thin-film transistors have a problem in that the dynamic range where A/D conversion is possible is significantly narrow.
FIG. 14 is a graph showing output characteristics of transistors. A curve (A) indicates output characteristics of a MOS device formed on a single-crystal silicon device, and a curve (B) indicates output characteristics of a polysilicon thin-film transistor device. Substantially horizontal portions on the curves (A) and (B) each correspond to a saturation region which satisfies equation (1) and in which Vgs>Vth and Vgs−Vth<Vds<Vkink, where Vkink is the voltage at which a kink phenomenon starts to occur. The region in which Vds>Vkink, does not satisfy equation (1). In the MOS device formed on the single-crystal silicon device, Vkink is high and equation (1) is satisfied in a relatively wide range. (That is, the horizontal portion on the curve is large.) Consequently, the range of the current Isense that can be derived from equation (6) is relatively large.
For example, in a device with a Vkink of 10 V, Vds(=VA−VS) must be less than 10 V. Assuming that the channel width W is 10 μm, the channel length L is 6 μm, the mobility is 1,300 cm2/V/S, and the thickness of the gate oxide film is 100 nm, Isense is calculated to be less than about 3 mA from equation (4). With respect to the lower limit, theoretically, when Vgs→Vth(Va→Vth−Vs), Ids→0. However, in practice, since a slight leakage current always flows, even if Vgs→Vth, Ids→Ileak, and no change is observed in a range lower than a certain level. Furthermore, in practice, taking the maximum range ΔVth of production variation of the threshold voltage Vth into account, VA must be approximately equal to or greater than ΔVth−VS. In consideration of all of the above, at Isense greater than about 1 nA, which is practical, and in a MOS device formed on a single-crystal device, it is possible to perform A/D conversion in an Isense range of 1 nA to 3 mA using the structure shown in FIG. 12. That is, the measurement dynamic range is about 3,000,000:1. Furthermore, in order to shift the range to the lower current side, the channel width W of the transistor 2 is decreased (or the channel length L is increased). In order to shift the range to the higher current side, the channel width W of the transistor 2 is increased (or the channel length L is decreased). In either case, the dynamic range does not change.
However, in a polysilicon thin-film transistor, in particular, in a low-temperature polysilicon (LTPS) thin-film transistor which is formed by a low-temperature process at 600° C. or lower, the mobility is about 100 cm2/V/S, and the kink voltage is low. A kink phenomenon starts to occur at a Vkink of about 6 V. Furthermore, the off-leak current is increased, and when Vds=Vgs=Vth, Ileak is about 10 nA (W=10 μm and L=6 μm). The maximum range ΔVth is also large being from several tens of millivolts to about 200 mV. The calculations performed in the same manner as above show that the measurable current range is 10 nA to 80 μA in terms of the current Isense, and the dynamic range is about 8,000:1, which is considerably lower than that of the MOS transistor.
As described above, in the A/D conversion circuit including polysilicon thin-film transistors, it is not possible to obtain a sufficient A/D conversion dynamic range compared with the case in which MOS transistors on a single-crystal silicon device are used, which is a problem.
JP-A-6-245152 is an example of related art.